build: Fix a misnamed variable in glib-tap.mk
build: Fix a misnamed variable in glib-tap.mk

This was causing anything listed in $(installed_test_extra_scripts) to
not be installed.

https://bugzilla.gnome.org/show_bug.cgi?id=766570

endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib